The cyclic AMP receptor protein (CRP) of Escherichia coli is a DNA binding protein that can activate transcription when bound to specific sites near promoters. We are studying a number of aspects of the mechanism and consequences of action of CRP on transcription. To study the cAMP-induced allosteric change, we have isolated allosteric-defective crp mutants and intragenic suppressors of that class. We conclude from in vivo and in vitro work with these mutant CRPs that the hinge of CRP and the part of the protein immediately adjacent to the DNA binding region are important in the cAMP-induced allosteric change. We have found that under certain conditions, CRP binding can cause a B to A-form transition in DNA. This could have implications in determining how CRP activates transcription. From our work, it appears that CRP induces a change in the DNA that might be transmitted to the promoter. By introducing changes in the DNA between where CRP binds and the promoter, we can affect CRP action: a nick or single-base gap hardly perturbs activation, a two-base gap has lower activity, and a four-base gap is non-functional. We have found that the pts operon which encodes proteins necessary for certain sugar uptake is regulated in a novel way by CRP. There are two CRP binding sites and there may be interaction between CRPs bound at the two sites.